Various types of PM sensors (i.e., particulate matter detection sensors) for detecting the amount of PM exhausted from an engine or the like have been proposed. For example, a PM sensor disclosed in JP-A-59-196453 (corresponding to U.S. Pat. No. 4,656,832) includes a pair of opposed electrodes on an insulating substrate. The accumulation of PM changes a resistance between the pair of the electrodes. By using this property, the PM sensor is configured to detect the amount of PM by measuring the resistance between the electrodes. In such case, a signal output circuit connected to a sensor element is a voltage-dividing circuit formed by a resistance between the pair of opposed electrodes and a predetermined shunt resistance. The signal output circuit is configured to output a voltage at an intermediate point of the voltage-dividing circuit as a sensor detection signal.
In addition, the PM sensor described above has a heating means such as a heater for forcibly burning and removing PM accumulated between the pair of opposed electrodes.
When the engine starts in a low temperature environment for a cold start period, water in the exhaust passage, in which a PM sensor is provided, tends to be attached to the PM sensor and other exhaust sensors. More practically, water formed by a combustion reaction of fuel and air is included in the exhaust gas from the engine, and water contained in the exhaust gas during the cold start period of the engine is cooled in the exhaust passage and condensed. Further, water, generated from the previous engine operation and remaining in the exhaust passage may be scattered and may fall on the sensors. When water falls on or attaches to the PM sensor, resistance between the electrodes changes due to the attached water, and thereby the amount of PM is incorrectly detected in some cases. In addition, an insulating substrate of the PM sensor may be damaged due to a partial cooling of the substrate at water attached portions, when the heater heats the sensor in a water attached state.